Backport of:
https://github.com/PowerDNS/pdns/commit/495f8e5f1f2c147f7431c5a6bfd4f4606b640fe3

From: =?UTF-8?q?Holger=20Hoffst=C3=A4tte?= <holger@applied-asynchrony.com>
Date: Thu, 9 Oct 2025 22:04:07 +0200
Subject: [PATCH] dnsdist: add fast path to roundrobin load balancing policy

There is no need to collect all servers that are up when the current
server is already a good candidate. This avoids needless heap allocation
and deallocation in the vast majority of cases.

Signed-off-by: Holger Hoffstätte <holger@applied-asynchrony.com>
--- a/dnsdist-lbpolicies.cc
+++ b/dnsdist-lbpolicies.cc
@@ -237,6 +237,14 @@ shared_ptr<DownstreamState> roundrobin(c
     return shared_ptr<DownstreamState>();
   }
 
+  static std::atomic<unsigned int> counter{0};
+
+  size_t serverIdx = (counter++) % servers.size();
+  shared_ptr<DownstreamState> serverState = servers.at(serverIdx).second;
+  if (serverState->isUp()) {
+    return serverState;
+  }
+
   vector<size_t> candidates;
   candidates.reserve(servers.size());
 
@@ -255,8 +264,7 @@ shared_ptr<DownstreamState> roundrobin(c
     }
   }
 
-  static std::atomic<unsigned int> counter{0};
-  return servers.at(candidates.at((counter++) % candidates.size()) - 1).second;
+  return servers.at(candidates.at(counter % candidates.size()) - 1).second;
 }
 
 shared_ptr<DownstreamState> orderedWrandUntag(const ServerPolicy::NumberedServerVector& servers, const DNSQuestion* dnsq)